The two major objectives of the proposed project are: a) To study the pathogenesis of human and experimental nephritis and particularly to explore the hypothesis that the two main pathogenetic mechanisms involved in production of glomerular injury, may also be responsible for interstitial nephritis and for immunologic damage in other parts of the nephron. (b) To study the pathogenesis of renal lesions in human as well as in experimental renal transplants. At least three possible mechanisms of immunologic injury will be investigated: 1) Injury produced by an immune response to transplantation antigens (rejection): 2) Injury produced in the graft, as well as in the host's kidney, by a reaction to antigens shared by donor and recipient (autoimmunity) and, 3) Injury produced by a transmission to the graft of the original nephritis of the donor (recurrence).